A 38-year-old man from Delhi presents with a 2-week history of right upper quadrant pain, nausea, and dark urine. He denies fever. On examination, he is mildly icteric. Laboratory tests show total bilirubin 3.5 mg/dL, direct bilirubin 2.8 mg/dL, ALT 280 IU/L, AST 240 IU/L, and alkaline phosphatase 180 IU/L. Transabdominal ultrasound shows a normal-sized gallbladder with no stones, normal liver echotexture, and no intrahepatic bile duct dilatation. The common bile duct measures 8 mm in diameter. Magnetic resonance cholangiopancreatography (MRCP) reveals a 12 mm filling defect within the distal common bile duct with proximal duct dilatation. What is the most likely diagnosis?

Explanation

## Clinical and Imaging Analysis **Key Point:** A filling defect within the common bile duct on MRCP is pathognomonic for choledocholithiasis. The normal gallbladder (no stones) indicates the stone migrated from the gallbladder into the CBD—a common complication of cholelithiasis. ### Diagnostic Criteria for Choledocholithiasis | Criterion | Finding in This Case | Interpretation | |-----------|----------------------|-----------------| | **CBD diameter** | 8 mm (normal to slightly dilated) | Obstruction from stone | | **MRCP finding** | 12 mm filling defect in distal CBD | Stone (radiolucent on CT, filling defect on MRCP) | | **Proximal dilatation** | Present | Obstruction distal to defect | | **Gallbladder** | No stones, normal wall | Stone has migrated; no acute cholecystitis | | **Liver echotexture** | Normal | No cirrhosis or parenchymal disease | | **Bilirubin pattern** | Predominantly direct (obstructive) | Biliary obstruction | | **Transaminases** | Mildly elevated (ALT > AST) | Hepatic inflammation from obstruction | ### Why MRCP Is the Gold Standard **High-Yield:** MRCP is non-invasive, does not require contrast injection, and has >95% sensitivity for detecting CBD stones. Stones appear as filling defects (dark signal) within the bright (T2-weighted) bile duct lumen. **Clinical Pearl:** Choledocholithiasis occurs in 10–15% of patients with gallstones. Risk factors include: - Large stones (>15 mm) - Multiple stones - Dilated CBD (>10 mm) - Age >65 years - Microlithiasis (bile sludge) ### Pathophysiology of Obstructive Jaundice ```mermaid flowchart TD A[Stone in distal CBD]:::outcome --> B[Obstruction to bile flow]:::action B --> C[Increased intraluminal pressure]:::action C --> D[Bile regurgitation into hepatocytes]:::action D --> E[Conjugated hyperbilirubinemia]:::outcome D --> F[Hepatic inflammation]:::outcome B --> G[Proximal CBD/intrahepatic duct dilatation]:::outcome ``` **Mnemonic: CHOLEDOCHOLITHIASIS IMAGING — "STONE IN DUCT"** - **S**tone (filling defect on MRCP) - **T**hick CBD wall (if infected/pancreatitis) - **O**bstruction (proximal dilatation) - **N**ormal gallbladder (stone has migrated) - **E**levated direct bilirubin - **I**ntrahepatic duct dilatation (if complete obstruction) - **N**o fever (unless ascending cholangitis) - **D**iameter CBD >8 mm - **U**ltrasound or MRCP confirms diagnosis - **C**onjugated hyperbilirubinemia - **T**ransaminases mildly elevated [cite:Robbins 10e Ch 18] ## Why Other Options Are Incorrect ### Acute Pancreatitis Acute pancreatitis can cause transient CBD edema and mild obstruction, but MRCP would show a **smooth, tapered narrowing** of the distal CBD (from edema), not a discrete filling defect. Additionally, amylase and lipase would be markedly elevated (>3× upper limit of normal), which is not mentioned here. ### Ampullary Adenocarcinoma Ampullary tumors cause a **shouldering** or **apple-core** appearance on MRCP with irregular, infiltrative narrowing of the distal CBD and pancreatic duct (double duct sign). A discrete, round filling defect is not typical. The patient would have progressive weight loss and more severe jaundice. ### Primary Sclerosing Cholangitis (PSC) PSC presents with **multiple strictures and dilations** ("beads-on-a-string" appearance) affecting both intrahepatic and extrahepatic ducts. A single discrete filling defect is not characteristic. PSC is associated with inflammatory bowel disease and presents insidiously.